The mature T-cell receptor (TCR) repertoire is formed during thymic development to fulfill two criteria: 1) peripheral T-cells must recognize foreign antigens associated with self MHC molecules, and 2) potentially harmful activity of T-cells specific for self-peptide/MHC complexes must be prevented. Tolerance to self may be achieved by physical deletion or by a variety of silencing mechanisms. In mature T-cells, the MHC restriction of the TCR matches the expression of the co-receptor molecule - MHC class I restricted TCRs are expressed by CD8+ T-cells, while MHC class II restricted TCRs are expressed by CD4+ T-cells. The aim of this application is to test if instead of CD4+CD8+ thymocyte deletion self MHC class II molecules can alter the lineage commitment and the CD8+ cell function. Preliminary data demonstrate that transgenic TCR specific for the H-Y antigen presented by H-2Db class I cross-reacts to self MHC class II. By breeding the H-Y TCR transgenic mice to background lacking or not H-21Ab, and by forcing the expression of CD4 in these backgrounds, it will be determined whether H-Y TCR-H-21Ab interaction during thymocyte maturation induces CD4+CD8- thymocyte deletion, endogenous TCR alpha rearrangement or functional unresponsiveness. To test if tolerance may obscure the stochastic component of lineage commitment, CD8 transgene will be introduced to H-Y mice on backgrounds lacking, or not, H-2IAb. To ask if tolerance can directly influence the lineage commitment the CD4 and CD8 gene shut-off in various transitional thymocyte populations on backgrounds lacking, or not, H-2IAb will be assessed. Testing functions of CD8+ T-cells in vitro and in vivo in H-Y mice on backgrounds lacking, or not, H2IAb or CD4 genes will determine whether tolerance to self MHC class II can alter the functional capacity of CD8+ T-cells and whether CD4 expression is required for the induction of tolerance, respectively. The proposed experiments should demonstrate for the first time that tolerance to self MHC class II molecules can affect the lineage commitment of thymocytes and the function of MHC class I restricted CD8+ cells. These findings should greatly contribute to our knowledge of the functional TCR repertoire formation and could be used to improve the immune defense against infectious agents and tumors.